The present invention concerns waveguides which transmit electromagnetic energy, e.g. visible, UV or IR radiation; such waveguides are usable as optical probes in devices for spectroscopically analyzing chemical or biochemical substances which, when it contact with the probe, modify some transmission parameters of this radiation in the waveguide. A measure of said parameters variation, for instance light absorption, diffraction, scattering, shift of the wavelength, fluorescence generation, etc. provides data directly related to the substance(s) to be analyzed. The latter may, either be dissolved in a liquid with which the waveguide is contacted, whereby the measurements will involve the bulk of the analyte, or it may react with a specific ligand applied to the guide surface, whereby a coating is formed whose rate of growth with time constitutes a distinctive feature of the substance to be analyzed.
Generally, such waveguides are laminar, flat or cylindrical fiber-like bodies and they assure the transmission of the light signal by multiple internal reflection at an angle of incidence .theta. larger than the critical angle .theta..sub.c (below which refraction occurs) but generally very close thereto.
An analytical device incorporating such kind of waveguides has been disclosed recently (see European Patent Application No. EP-A-84.810.601.9) as well as the measuring techniques pertaining to such apparatus. The waveguides disclosed in the present invention can be used in the said apparatus and constitute, indeed, particularly useful embodiments thereof. In the desciption that follows, direct reference to the said document will be made, the present waveguide being considered as directly usable as a component of the analytical apparatus disclosed and claimed therein.
The waveguide disclosed in the reference document comports a dual optical transmission element (51, 52; 71, 72; see FIGS. 1-3) whose geometry requires using, for injecting a light signal therein at an appropriate angle of incidence and to collect the signal exiting therefrom, optical means (mirrors) arranged in a relatively complicated relation. Indeed, because of the particular orientation of the input and output prisms of said optical elements (see FIGS. 1 to 3 of the reference document), the path of the light signals outside said elements is rather intricate and makes sharp angles with the direction of the waveguide. It was therefore desirable, for obvious constructive reasons, to provide a waveguide enabling the injection of an input signal in a direction generally parallel to its optical axis or at a substantially small angle to this direction, the output signal obeying roughly the same criteria.
Claim 1 discloses a waveguide in conformity with the above requirements. According to a preferred embodiment thereof, the angle (.beta.) between the slant plane of the wedge bevelled volume and the parallel surfaces of the waveguide corresponds to 0.5 (90.degree.-.theta.).
Regarding the input and output signals, it should be noted that the output signal can in some cases derive directly from the input signal (or is a residue thereof) after this signal has been modified along the waveguide (for instance, partly absorbed) by the action of the medium in which the guide is placed; otherwise the output signal can consists of a radiation of a different kind (e.g. production of fluorescence) generated by the medium upon interaction with the evanescent wave component of the excitation signal. Such a fluorescence is then returned into the guide at the interface between the waveguide and the analyte and is transmitted therein at an angle of internal reflection which may or may not correspond to the angle .theta. of the excitation signal. If the difference is significant, the direction of the fluorescent ouput signal emerging from the guide may diverge from that of the input signal (this can for instance be of significance when the same prism is used as the input and the output) which situation may be constructively advantageous, it being possible to angularly offset the output detector relative to the position of the input source. Evidently, the output prism can be made so as not to coincide with the input prism and can be placed elsewhere on the guide relatively to the position of the input prism. For instance, the input prism can be at one end of the waveguide and the output prism can be at the other end. In this case the bevel angles of the two prisms can be alike or they can have different values, this being particularly the case if the excitation and response signal travel at different .theta. angles.
It is further remarked that, depending on the desired waveguide embodiments, the wedge-shaped portion of the guide can be singly bevelled or doubly bevelled. In other terms, in the case of a single bevel, only one of the main faces of the guide starts diverging from the other toward the end of the guide, the second of the main surfaces keeping parallel to the guide optical axis; in the other case, the second face also diverges in a manner symmetrical or unsymmetrical with the first one.